Distribution and availability of cadmium in profile and aggregates of a paddy soil with 30-year fertilization and its impact on Cd accumulation in rice plant.

The research was conducted to investigate the accumulation, distribution and availability of Cd in paddy soil and their relation to Cd in rice plants under 30-year fertilization regimes. Six treatments were involved in the study: control without fertilization (CK), chemical fertilizer (NPK), high nitrogen chemical fertilizer (HN), rice straw incorporation (ST), low and high dosage of manure fertilizer (LM and HM). Total and DTPA extractable concentration of Cd (T-Cd and DTPA-Cd) in bulk soils (20 cm topsoil), profiles (0-60 cm) and aggregates (>2, 1-2, 0.5-1, 0.25-0.5, 0.053-0.25 and < 0.053 mm) were investigated. The Cd concentration in relevant rice plant (roots, stems, leaves, husks and grains) were also analyzed. Manure fertilizers caused T-Cd accumulation in bulk soil with a significant increase of 36.2% in LM and 81.2% in HM. Similar impacts of manure fertilizers were observed in DTPA-Cd in the bulk soil. Further, the HM generated a further accumulation in deeper soil layers, presenting a remarkable increase of T-Cd (28.3%-225%) in 10-40 cm and DTPA-Cd (116%-158%) in 10-30 cm profiles. Moreover, the continuous application of manure fertilizers enhanced the availability of Cd in all aggregate size classes with an increase of 17.3%-87.8% in DTPA-Cd. Organic fertilizers (LM, HM and ST) heightened the content of Cd (38.0%-152%) in all parts of rice plant. The accumulation of Cd in rice plants was directly affected by fertilization regimes and Cd availability in the 10-20 cm soil layers and 0.25-0.5 mm aggregates. In conclusion, long-term application of manures resulted in increasing availability of Cd in aggregates and in topsoil and subsoil layers, which accordingly enhanced the accumulation of Cd in rice plants.